VALIDATIONS

Hi there.
I really enjoy the use of your plugin but I would like to know how can I apply validations to a field.

Let’s say that for the shipping, my clients can use their own UPS account instead of having me charging them to use my own account.

So I created a box where they just type their account number.
How could I validate this data?

Let’s say UPS account numbers are 6 characters, so I wouldn’t want them to type more or less numbers.

Thank you so much in advance.